1. Field of the Invention
The present invention relates to audio devices for vehicles and in particular to audio devices including vibrators that are mounted to interior trim panels of vehicles, which interior trim panels at least partially define the vehicle cabins, so that sound is produced due to vibrations applied to the interior trim panels by the vibrators.
2. Description of the Related Art
A known vehicle audio device of a type described above is disclosed in PCT International Publication WO/01/15489 as electromagnetic transducer assemblies in the form of sub-assembled drive motors. One of the sub-assembled drive motors is shown in FIGS. 5(A) and 5(B) and is labeled with reference numeral 90. The drive motor 90 generally includes a motor base 96 and a vibrator 92 coupled to the motor base 96. The motor base 96 is secured to a headliner 94 defining a surface of a vehicle cabin. A steel cap and a magnet configure the vibrator 92. The motor base 96 includes a tubular portion 96t and a flange 96f. The tubular portion 96t has an outer peripheral surface with external threads 96m. The flange 96f extends outward from the base end of the motor base 96. The motor base 96 is secured to the headliner 94 via the flange 96f. 
The vibrator 92 further includes a ring-shaped guide member 93 disposed at a lower end of the vibrator 92. The guide member 93 has an inner peripheral surface with internal threads 93w. The vibrator 92 may therefore be mounted to the head liner 94 by engaging the internal threads 93w of the guide member 93 with the external threads 96m of the motor base 96 and thereafter tightening the internal threads 93w until the guide member 93 contacts the flange 96f of the motor base 96.
However, using this mounting structure of the vibrator 92 to the headliner 94 through the tightening of the internal threads 93w against the external threads 96m makes it likely that the tightened condition cannot be maintained due to potential loosening of the threaded connection during a long period of use. In particular, if the mount base 96 or the guide member 93 is made of resin, such loosening may easily occur when the temperature varies within the vehicle cabin, since the resin material typically has a large linear coefficient of thermal expansion. When loosening occurs, a clearance may be produced between the external threads 96m and the internal threads 93w, causing insufficient transmission of the audio vibration produced by a voice coil 92b of the vibrator 92. Consequently, the desired sound may not be effectively produced.